In recent years, the semiconductor integrated circuit elements (IC chips) used for a CPU in a computer or the like operate at higher speeds, and with higher level functional features, than previously. Along with this advance, the number of terminals has increased and the terminal interval has tended to be smaller or more narrow. Generally speaking, terminals in large numbers are densely packed on the bottom surface of the IC chip in an array, and a terminal block so formed is connected to a terminal block at the motherboard side by a conventional “flip chip” connection. However, because there is a large difference in pitch between the terminal block at the IC chip side and the terminal block at the motherboard side, it is difficult to directly connect the IC chip to the motherboard. Therefore, the connection method generally employed is one wherein a package constituted by the IC chip, which is mounted on a wiring board for mounting the IC chip, is produced and then mounted on the motherboard. Examples of such wiring boards which constitute this type of package include a wiring board wherein a ceramic chip is embedded in a core substrate comprised of a polymer material so as to form a core portion, and a built-up layer is formed on both the top and rear surfaces of the core portion (e.g., see Japanese Patent Application Laid-Open (kokai) No. 2005-39243).
Recently, the strong demand for a system capable of offering higher performance than a package including only a single microprocessor has been increasing. One example of such a system is a package including a plurality of microprocessors. This type of package includes a multi-microprocessor structure which is capable of performing parallel processing of plural threads (tasks), something which a single microprocessor structure cannot perform, and results in improving the processing capacity of the entire system. Further, the multi-microprocessor structure has improved failure-resistance as compared with the single microprocessor structure.
When implementing a package having a multi-microprocessor structure, for example, the size of a ceramic capacitor of the wiring board corresponding to that disclosed in the above-identified reference is enlarged, and microprocessors are mounted at plural locations within an accommodation area of the enlarged ceramic capacitor. However, with this structure, there is a decrease in the available area in a substrate core for forming a conductor portion, such as a through-hole and a conductive pattern. Thus, wiring of the conductor portion used in electrically connecting the top and rear surfaces of a core portion presents difficulties, with the result that the overall size of the wiring board is unlikely to be decreased, as is desirable.
Further, the wiring board according to the above-described reference can be used as a component part of a package having a multi-microprocessor structure when the need for a plurality of power supply systems for the microprocessors can be met by a shared power supply system. However, when the power supply system cannot be a shared system, and it is thus necessary to establish a power supply system for every microprocessor, and each microprocessor cannot be fully operational, even though the wiring board is used as a component part of the overall package. Therefore, the advantages of a multi-microprocessor structure cannot be fully realized.
In this type of package, because the calorific power of the microprocessor also increases, it is very important to match the coefficients of thermal expansion of the microprocessor and the wiring board. In this regard, when the thermal expansion coefficients of the microprocessor and the wiring board do not match, there will be substantial heat stress imposed on the corresponding microprocessor, and cracking and a faulty connection may occur in the microprocessor. Therefore, it is necessary to adopt a construction which can reduce the influence of such heat stress.